Your contract seems working on Remix VM. Are you sure that you transferred the tokens to DappTokenSale contract? As an alternative, you can directly deploy contract in your TokenSale contract using tokenContract = new Edcoin(params)
– Mehmet DoğanApr 18 at 7:22

No i haven't transferred tokens to Sale contract. How to do it? I deployed Token contract, supplied its address to sale contract while deploying it.
– SaeediApr 18 at 7:33

make following call to Token Contract : >>>>>>transferN(tokenSaleAddress, totalSupply) from the account that deployed token contract.
– Mehmet DoğanApr 19 at 11:06

Transferred it but the error is still persisting
– SaeediApr 22 at 5:22